main shutoff valve; stopcock
Refers to the main valve that controls the flow of gas or water into a building. Often used in contexts of turning off the supply for safety or maintenance.
After an earthquake, please first turn off the main gas valve.
When you're away from home for a long time, it's better to shut off the main water valve.
Compound of 元 (moto, 'origin, source') and 栓 (sen, 'plug, stopper, valve'). The kanji combination directly conveys the meaning of the main valve at the source of a utility line.